The invention relates to a powder coating method for a metallic surface which has been provided with an oil film, wherein a powder layer is deposited on the surface, melted by heat treatment and subsequently hardened by cooling. Such a method is employed for coating metal parts as used in the automotive and other engineering industries, for casings of home appliances and other devices, for metal furniture and shelving, and for toys, for example.
Manufacturing these metal parts usually starts out from coiled band steel which has been oiled, painted or otherwise coated for protection against corrosion during transportation and storage ("coil coating"). The band steel is reeled off the coil and further worked such as by sawing, punching, pressing, drilling, folding and bending to obtain the desired product. During these processing steps, the oil or paint coating also serves as a lubricant and release agent between the tool and the workpiece (sheet or band metal).
When working from an oiled coil, the finished parts are painted or coated for protection against corrosion and for obtaining the desired colour. In wet painting, a paint containing a solvent is used. However, the evaporation of the solvent causes severe environmental problems.
These problems can be avoided by using an electrostatic powder coating method such as described in DE 3,838,928 Al. An electrostatically charged powder is sprayed onto the surface to which it adheres by electrostatic forces. Suitable powders are made of thermoplastic resins, e.g. polyester or epoxy resins or mixtures thereof. The powder layer is melted to form a viscous substance which adheres well to the metal and results in a smooth surface. The coating is hardened by cooling.
To obtain a high quality surface, cleaning of the metallic surface for removing the oil prior to the powder coating step has been considered absolutely necessary in the art. This cleaning step, however, requires the use detergents and solvents which again constitute environmental burdens, and further causes substantial costs. Costs are involved not only in the equipment required but also in the energy used to dry the metallic surfaces that have been wetted by the detergent or solvent. No high performance would be possible without drying.
These problems could be avoided by using pre-coated or painted coils instead of oiled ones. This technique, however, has severe disadvantages in that large storing area is required for making products in a variety of different colours. Further, changing the colour always requires a cumbersome change of coils at the production line. As another difficulty, colours of different coils are never identical, particularly with coils from different suppliers.
In addition, when using pre-coated or painted band or sheet steel, the edges of the metal parts subjected to sawing, punching or drilling are not coated and are therefore susceptible to corrosion. Further, since these parts have sharp edges their handling implies the danger of injury.
The above disadvantages can be avoided by using oiled coils and coating the parts after all mechanical working steps have been completed. Electrostatic powder coating is particularly suitable because the electric flux lines can be directed so that a particularly large amount of powder will adhere to the edges of the workpieces. Therefore, the resin coating formed on these areas after hardening is particularly thick and results in rounded edges. The main disadvantage of this method, however, resides in the costs and environmental problems which occur when the oil is removed, as described above.